Booking an ex-BUD TATL economy roundtrip with layovers in LHR each direction and have the option to book with either AA (and have the intra-Europe BA segments as AA codeshares) or BA (and have the overwater AA segments as BA codeshares). I'm an AA member with no status. My quick research makes it look like I'll earn slightly more miles by booking with BA since I'll earn based on mileage instead of fare cost, but are there any downsides to booking with BA? Would probably be worth giving up those extra miles and booking with AA if it meant I could actually select seats for the overwater segments, for example.
Thanks!